1541 잃어버린 괄호

#include <stdio.h>
#include <string.h>
 
int main() {
    char arr[51];
    scanf("%s", &arr);
 
    int flag = 0, num = 0, rst = 0, minus = 0, prev = 0;
    for (int i = 0; i <= strlen(arr); i++) {
        if ('0' <= arr[i] && arr[i] <= '9') {
            num *= 10;
            num += (arr[i] - 48);
        }
        else {
            if (flag == 0)
                rst += num;
            else
                minus += num;
 
            num = 0;
 
            if (arr[i] != '+') {
                if (flag == 0)
                    flag = 1;
                else {
                    rst -= minus;
 
                    minus = 0;
                    flag = 1;
                }
            }
        }
    }
    printf("%d", rst);
}